Development of an AlN Deep UV Detector for Space Application
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Development of an AlN Deep UV Detector for Space Application Feng Zhong, Changhe Huang, Yuri V. Danylyuk, and Gregory W. Auner, Department of Electrical and Computer Engineering, Wayne State University, Detroit, MI ABSTRACT AlN is a very promising deep UV sensing material suitable for space application such as exoatomospheric solar blind detectors due to its large band gap (6.2 eV), excellent radiation and thermal stability. In this paper, a deep UV detector based on AlN photoconductor was successfully grown and characterized. High quality AlN thin films on Sapphire have been confirmed by in-stu RHEED and X-ray diffraction measurements. The film has an optical Eg of 5.96 eV. The detector has extremely low dark current. Different electrodes were investigated as contact materials. Al and Pt are more suitable electrode contacts for AlN photoconductor compared to Ti. The DC responsivity of the detector starts to rise for photon energies of about 255 nm (4.9eV) and reaches its maximum 7×10-5 A/W at 195 nm (6.5eV). INTRODUCTION AlN is a very promising deep UV sensing material suitable for space application due to its large band gap (6.2 eV), excellent radiation and thermal stability. Recently, many researchers have developed UV detectors based on GaN and AlGaN alloy [1,2]. These detectors are visible or even solar blind detectors due to the wide band gap of III nitrides. There is also great interest in detectors for space applications operating in the deep UV to soft X-ray (250-100 nm) region of the spectra. However, It has been shown [3, 6] that the responsivity of detectors decreases dramatically with the increase of Al composition in the AlGaN alloy, which corresponds to the deep UV region. So far there is no report on AlN based UV detector. In this paper we report the preliminary result of a deep UV detector based on AlN photoconductor. EXPERIMENT AlN photoconductors were grown at the Wayne State University Center for Smart Sensors and Integrated Microsystems by Plasma Source Molecular Beam Epitaxy (PSMBE). ). PSMBE uses a unique hollow cathode plasma source lined with MBE grade aluminum. High quality AlN epitaxial layers have been grown by using this system [4, 5]. The base pressure of the system is maintained in the range of 10-9-10-10 Torr. Radio Frequency (RF) power is supplied to generate plasma inside the hollow cathode. The dynamic pressure during deposition is maintained at 1× 10-3 Torr. Negative acceleration bias is applied to the substrate. The growth temperature, acceleration bias, N2 flow and RF power level can be adjusted to get optimum thin films. The growth was conducted on C-plane Sapphire. Prior to the growth the Sapphire substrates were ultrasonically cleaned and etched to get atomic smooth surface. A thin (~500 Å) low temperature (400 °C ) buffer layer was deposited first. The final growth was performed at 650 °C. RF source power was 200 W. The N2 and Ar flow ratio was kept at 10/40 sccm. The thickness of films ranged from 0.2 – 1 µm. The testing device has a structure of a simple interlaced metal e
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